The present invention relates to a transparent hardcoat film having high hardness and an antireflection film, and more particularly to a hardcoat film and an antireflection film which, in use, are laminated onto the surface of displays, such as CRT, LCD, and PDP.
In recent years, plastic products are being substituted for glass products from the viewpoints of fabricability and weight reduction. Since, however, the surface of the plastic products is likely to be scratched, in use, a hardcoat film is in many cases laminated onto the plastic products to impart scratch resistance thereto. Also in the case of the conventional glass products, there is a growing tendency for a plastic film to be laminated thereonto to prevent glass pieces from being scattered upon breaking of the glass products. Since, however, the hardness of the plastic is unsatisfactory, a hardcoat is in most cases formed on the surface of the plastic films.
Further, in recent years, displays, such as CRT and LCD, have been widely spread, and there is an ever-increasing tendency for a hardcoat film to be laminated onto the displays to protect the surface of display images. For the same reason as described above, an increase in hardness is required, and, at the same time, good visibility of the display screen per se through the hardcoat film is required. A known method for forming a hardcoat to eliminate the disadvantage of the surface hardness is to coat a thermosetting resin or an ionizing radiation-curable resin, such as an ultraviolet-curable resin, onto a base substrate followed by curing to form one layer of an about 3 to 10 xcexcm-thick coating.
In this coating thickness, however, the coating is likely to be influenced by the deformation of the base substrate, and thus, the film strength is not satisfactorily high. Increasing the modulus of elasticity of the hardcoat is effective in suppressing the deformation of the substrate. In this case, however, the hardcoat is likely to be cracked, and, in addition, curling is increased due to cure shrinkage of the hardcoat. At the same time, due to stress concentration on the surface of the hardcoat, stress strain is likely to cause cracking and consequently is likely to cause a scratch on the surface of the film. Although the hardness can be increased simply by increasing the thickness of the hardcoat, this cannot prevent cracking or separation and curling of the hardcoat.
In order to solve these problems, Japanese Patent Laid-Open Publication Nos. 93545/1992 and 8350/1993 have proposed a method wherein a multi-layer hardcoat of two hardcoats is formed in such a manner that the hardcoat provided on the surface layer side has higher hardness (higher modulus of elasticity) than the hardcoat provided on the substrate side to improve the surface hardness of the hardcoat film. Higher modulus of elasticity in the hardcoat on the surface layer side can certainly prevent the hardcoat from being scratched by materials having relatively low hardness, such as pencils, and, hence, can provide an effect associated with improved hardness.
However, against materials, which as such have high hardness, such as the tip of a mechanical pencil or a diamond needle, excessively high modulus of elasticity of the hardcoat causes stress concentration on the surface of the hardcoat. This is likely to cause a scratch on the surface thereof. In the case of a hardcoat film wherein an antireflection layer (AR layer) has been formed by vapor deposition or sputtering onto the multi-layer hardcoat having the above layer construction, this tendency is significant, and stress concentration on the AR layer to cause damage is remarkable and visually observed.
Accordingly, it is an object of the present invention to provide a hardcoat film comprising a substrate having thereon two or more hardcoats, which hardcoat film has improved surface hardness, does not undergo stress concentration-induced damage, and is less likely to be scratched.
In order to attain the above object of the present invention, according to one aspect of the present invention, there is provided a hardcoat film comprising: a transparent substrate; and a multi-layer hardcoat layer provided on at least one side of the transparent substrate, the multi-layer hardcoat layer comprising two or more hardcoats, the modulus of elasticity "sgr"m of a hardcoat provided closest to the transparent substrate being higher than the modulus of elasticity "sgr"s of a hardcoat provided as a surface layer. As used herein, the xe2x80x9chardcoat provided as a surface layerxe2x80x9d refers to at least one layer of the hardcoats other than the hardcoat provided closest to the transparent substrate.
In preferred embodiments, the present invention provides a hardcoat film wherein the modulus of elasticity "sgr"m of the hardcoat provided closest to the transparent substrate and the modulus of elasticity "sgr"s of the hardcoat provided as the surface layer satisfy a requirement represented by formula 3 greater than "sgr"mxe2x88x92"sgr"s greater than 0, a hardcoat film wherein the modulus of elasticity of the hardcoat provided closest to the transparent substrate is not less than 8 mN/xcexcm and not more than 13 mN/xcexcm and the modulus of elasticity of the hardcoat provided as the surface layer is not less than 6 mN/xcexcm and not more than 9 mN/xcexcm, and a hardcoat film wherein the modulus of elasticity "sgr"h of the hardcoat film and the modulus of elasticity "sgr"s of the hardcoat provided as the surface layer satisfy a requirement represented by formula 2xe2x89xa7"sgr"hxe2x88x92"sgr"sxe2x89xa70.
Further, in preferred embodiments, the present invention provides a hardcoat film which has a surface hardness of 4H or higher in terms of pencil hardness and a scratch hardness of not less than 150 g, a hardcoat film wherein at least the hardcoat provided closest to the transparent substrate contains 20 to 80% by mass of inorganic fine particles, and a hardcoat film wherein the content of the inorganic fine particles in at least the hardcoat provided closest to the transparent substrate is higher than the content of the inorganic fine particles in the hardcoat as the surface layer.
Furthermore, in preferred embodiments, the present invention provides a hardcoat film wherein the inorganic fine particles are ultrafine particles of silica having a particle diameter of not more than 100 nm, a hardcoat film wherein the multi-layer hardcoat layer has been formed from a material composed mainly of an ionizing radiation-curable resin, a hardcoat film wherein the total thickness of the hardcoats is 10 to 50 xcexcm, a hardcoat film wherein the transparent substrate has a surface hardness of not more than HB in terms of pencil hardness, and a hardcoat film wherein the transparent substrate is a polyethylene terephthalate (hereinafter referred to simply as xe2x80x9cPETxe2x80x9d) film having a thickness of 100 to 300 xcexcm.
According to another aspect of the present invention, there is provided an antireflection film comprising an antireflection layer provided on the multi-layer hardcoat layer in the above hardcoat film. In preferred embodiments, the present invention provides an antireflection film wherein the antireflection layer has a multi-layer structure of two or more layers formed by sputtering or vapor deposition, and an antireflection film wherein the antireflection layer in contact with the multi-layer hardcoat layer is formed of ultrafine particles of a metal oxide.